Telecommunication protocol and data formats for communication networks have been standardized world-wide. With this standardization, and under OSI (Open Systems Interconnection) protocol, communications networks composed of various types of hardware may communicate with each other.
With OSI, users of computer system can build the system most appropriate for their purpose. In other words, where before users often had problems connecting varying types of hardware to the network, OSI allows users to connect to a network regardless of the type of hardware they employ. In this context, users can integrate a total network system using various hardware, such as a combination of a work station made by "A company", a printer made by "B company", and a facsimile machine made by "C company", using the combination that best fits their needs.
The move toward network utilization has occurred first in the business world, and various network services are available for the business customer. One well-known example is online data retrieval from data bases over public telephone lines. However, the present invention concerns local area networks, which recently have attracted a good deal of public attention. "8000 INS" (XINS), which is sold by Fuji Xerox Corp., is representative of this type of network service wherein the functions performed by the hardware appear to the user to be network services. For example, these functions can be print servers, file servers, or mail servers.
A print server prints out a hard copy of the indicated text or data (with a laser printer, for example) in response to a command from a work station operating as a user terminal. A file server performs file handling, such as storing and sending indicated files to and from the customer, in response to a command from the work station operating as a user terminal. A mail server performs a mailing service by which private mail boxes are managed, such as writing a mail message that was sent from another station to the indicated address or such as sending a mail message that is required from the indicated mail box to another customer.
In the "8000 INS" system, the word "server" is used to described a combined "server" and "work station", thus indicating that specified services are supplied to a client on the work station of the network as a user. It is predicted that in the near future various kind of services, including those services described above, will be available in the network system.
Currently, to add a new service that has previously been unavailable in the network system, it is usually necessary to use a program as an interface for the newly added service. This program must be loaded on the work station at the client side. However, adding a new service to the network as described above requires some changes in the previously existing system. This requirement increases dependency on the previously existing system, decreases the flexibility of the total network system, and makes it difficult to have various kinds of services on the network system.
In this context, it becomes important to be able to add a new service while still maintaining the flexibility of the total network system to accept various kinds of services to be supplied. Herein, newly added services would not require any functional changes to an existing network system.
To realize the aforesaid requirement to maintain system flexibility, some systems have been proposed to add new services to the network without any new interface at the client side in the system.
One example of such a system is demonstrated in FIG. 6. It employs an interface for an electronic mailing system. In the figure, a client 12, such as a work station; a mail server 13; and a new server 14, by which newly added services are supplied, are connected to a bus cable 11. In this example, a mail box 15, which is used only by the new server 14, is prepared in the mutual mail server 13. When the client 12 sends an electronic mail message that concerns the new server 14, the mail message is stored in the mail box 15. On that occasion, among the indexes of the mail such as a title 17, a message 18, and data 19, which are listed on the electronic mail sheet 16, title 17 or message 18 is interpreted as a command for the service, and the service is executed for the associated data 19 or message 18.
The newly added server 14 checks if any mail has been sent to its own mail box 15 within a predetermined time interval, and when a mail message has been sent to it, executes a predetermined process for the mail message and stores the result of the process in a predetermined storage area or in a storage area indicated by the mail message.
A second example of such a system is demonstrated in FIG. 7. In the figure, the same reference numerals are used to show the same equipment of FIG. 6. For the system in FIG. 7, to file server 21 is employed instead of the mail server 13 in FIG. 6. The file server 21 includes a storage area 22 to serve the newly added service 14 corresponding to the server 14 in FIG. 6. The storage area 22 has a layered structure to match the services to be used with various stages.
The difference between the system of FIG. 7 and that of FIG. 6 is that the mail box 15 in FIG. 6 is theoretically prepared for each individual use, while in the system of FIG. 7, the data which are managed by the file server 21 can be possessed by multiple users because the file server 21 itself performs file handling.
A third system could be a combination of the first and second types of systems. In that system, a command to perform the ordered service for the indicated data in the specified file drawer and to store the result in the specified file drawer under the indicated directory is conveyed using electronic mail.
In the first proposed system of FIG. 6, data and commands are handled through the mail server 13. The structuring of the title 17 and messages 18 in the electronic mail sheet 16 matches the various services and does not require a special interface to use the newly added service for each client.
In this first proposed system, the newly added server 14 may cause a problem in the mail server 13 or may impede logical access to the mail server 13 because of, for example, errors in the program to be applied for the server 14. In case of such an accident, the mail box 15 of the server 14 may be destroyed which would affect not only the mail box 15 but the mail boxes for multiple users because there are many mail boxes collectively arranged with the aforesaid server 14 and the damage caused by error in the program may propagate to the other mail boxes therein. In other words, in the first proposed system, the more active the server 14 becomes, the more the service depends on the network, leading to decreased credibility of the system. The possibility of problems increases when the results of processed data are returned to the users.
The second proposed system has the same possible problems. In the file server 21, which can be accessed by multiple users, once a problem occurs, it tends to propagate to third parties who have no relation to the aforementioned server 14, causing damage such as lost data.
The third proposed system contains very sophisticated services, since the system itself is made up of a combination of the first and second systems. However, conversely, the factors that decrease credibility are increased in the system.
As described above, with regard to systems in which mail servers 13 or file servers 21 are used, close connection to the data communication decreases network dependability. At the same time, such connections makes it more difficult to add or to remove a server from the system once the system has been in operation. For example, the following describes an introduction of a new file server 21 to the second type of system. An operator of the system or a program in the system prepares a new memory area for the working area to be used for the new service in the file server 21, which has already been used. Even when the new file server 21 is prepared independently of the previously existing file server, the total number of the nodes becomes two. In short, because the cost and occupied space for the working area are increased, adding and removing a server is problematic.
The popularity of OSI requires dependability of each node in the network, since the propagation of damage by the interference of nodes is undesirable.